HYDROGEOLOGICAL INVESTIGATIONS OF THE DECCAN TERRAIN OF THE KOYNA SUB-BASIN, INDIA

Abstract

The western margin of the Indian Peninsula is characterised by hilly terrain, known as Western Ghats (hills). The Koyna sub-basin (a part of Krishna River basin) is located east of the main ridge, locally called the Sahyadri Hill Range. Trending north-south in general, this sub-basin with Koyna river and its tributaries covers an area of 2036 sq km in the Deccan Terrain of the district of Satara, Maharashtra State, India. This sub-basin drew attention of geoscientists after the Koyna earthquake (magnitude 7) of 1967. Since then, detailed geological, tectonic and seismic investigations of the sub-basin have been carried out by several workers to work out the causative factors which triggered the earthquake. Despite these investigations, very little studies have been done on the hydrogeological aspects of this sub-basin. The present work, therefore, is an attempt in this direction to evaluate the hydrogeological framework of the sub-basin and assess its groundwater potential. The annual normal rainfall, due mainly to monsoon, decreases steadily from western (6024 mm) to eastern parts (745 mm) in the area and shows wide variation due to orographic influence of the Western Ghats. The sub-basin is underlain by basaltic lava-flows capped by laterites on the flat topped hills at higher elevation. In the lower reaches of the river valleys alluvium is found. The soils are basically of two types, viz. lateritic soils and black cotton soils. Four distinct geomorphic units, viz. plateau (flat), dissected plateau, shallow buried pediment and deep buried pediment have been identified on the basis of thematic mapper data. The major lineaments, identified on the satellite imageries and aerial photographs, are found to trend northeast-southwest, north-south and northwest-southeast. The drainage, mostly controlled by lineaments, shows a sub-dendritic pattern. The Koyna river flows along a north-south trending lineament in the upper reaches. It trends east-west along several lineaments in the middle and lower reaches. Morphometric analysis puts Koyna as a 6th order stream with bifurcation ratio of 5.08 which also indicates structural control of the drainage network. The aquifers are found to be associated wi,h the basal*, laterites, alluvium, soil and talus deposus. The upper vesieular uni, of me basaltic flows, unlike the lower massive par, exhibits pnmary porosity and permeability and is weathered in most pans even at depth. However at many places, the non-vesicular (massive, uni, has developed secondary porosity due' to fractunng, jointing and weathering. m laterjtes ^ ^ ^^ ^ ^ ^ permeable, bu, have very low specific region of groundwa,er. The water bearing properties of ,he clay- rich alluvium are largely controlled by sand/clay ratio. The lateritic soils have be„er potential then that of black co„on soils. Based on ft. analysis of 545 wells, ,he shallow aquifers are found up,o amaximum depth of 25 m. The la,eri,es, alluvium, soil, talus deposits and the weathered uppermost basalts from shallow aquifers in the area. The deeper aquifers are found below the zone of weathering in basalts, due mainly to fracturing. The groundwater in deeper aquifers occurs at the contact between the basaltic flows wtthtn the vesicular and amygdaloidal section at the lava-flow top or within the fractured and jotnted section. I, occurs under semi- confined condition when me productive horizon is separated by fractured and or jointed basal,. Confined aquifer conditions are found when the wafer bearing horizon lies a, ft. contact 0f to0 ^1bm^K upper one^^u^ basal, and ft. lower one as vesicular or fracured basal,. A, many places shallow aquifers are connected to deeper aquifers through fractures. Well yields of groundwater are found to be related with the lineaments and the geomorphic features. While the lineaments have abetter control over the yields of the borewel.s tappmg deeper aquifers, the geomorphic features have greater control over the shallow aquifer system. Deep buried pediments and the NE-SW tending lineaments are highly potential areas for dugwells and borewells respectively. Borewells in deep buried pediments with NE-SW lineaments have yields as high as 39,000 Iph. Surface-water is lifted extensively for irrigation from the Koyna river through anumber of l,f,-,rr,gatio„ schemes which run throughout the year, h the command area of these hft-trngation schemes, .herefore, groundwater is poorly extracted due ,„ easy availability of surface-water for irrigation. Because of continual recharge due to irrigation, the water levels in this area are rising steadily. Thus, this is leading to near water-logging conditions in the lower reaches of the sub-basin. The non-command area, on the other hand, indicates a gradual decline of water levels due to overdraft for irrigation. The decline in water level is more pronounced in the southern parts away from the Command area, due not only to overdraft but also to the general declining trends of rainfall since 1974. In fact, drought occurs in this area once in every four years. This part of the sub-basin, therefore, calls for artificial recharge of the aquifers to augment the existing groundwater regime. The hilly region is dotted by many cold-water springs. The high rate of precipitation and the terrace-like topographic features found in the hills facilitate recharge of the aquifers through weathered/fractured/jointed rocks. These aquifers, whenever are dissected by fractures, joints and exposed to hill face, are drained in the from of springs. Based on the examination of 121 springs, it is found that they generally emerge through fractures or the contact between (i) laterite and lithomargic clay or poorly lateritised basalt, (ii) vesicular basalt and massive basalt, (iii) highly weathered massive basalt and moderately or poorly weathered massive basalt or redbole, (iv) talus deposits and hard massive basalt or laterite or lateritised basaltic flow. They are mostly distributed at an elevation range of 600 to 1350 m above MSL with maximum concentration (47%) in between 900 to 1000m elevation. The mean discharge of the individual springs in winter is about 46 m3/day as against the mean discharge of 28 mVday in summer. As per Meinzer's classification (1923), mostly the springs of magnitude 5 and 6 are found in the area. The springs have a recharge area of 722 sq.km and an yearly discharge of 14 MCM. Based on the nature of their emergence the springs could be classified as contact springs (89%) and fracture springs (11%). However, since the emergence of groundwater in the form of springs is largely controlled by the water-bearing properties of the formations in the study area, these springs can also be classified on the basis of their source-aquifers. Thus, a simple classification has been proposed which classifies the springs into five different types such as, (i) laterite springs (9%), (ii) talus springs (23%), (iii) vesicular basalt springs (20%), weathered non-vesicular basalt springs (37%) besides fracture springs (11%) associated with any type of aquifer. The first four types comes under the category of 'contact springs'. The transmissivity of the shallow aquifers as estimated through the Papadopulos-Cooper method (1967), Boulton-Streltsova method (1976) and Mishra-Chachadi method (1985) is found to be of the order of 128 nWday for talus deposits, 64 to 135 mVday for the highly weathered and jointed basalt and 57 mVday for the poorly weathered and poorly jointed basalt. The dugwells were tested in terms of productivity with the help of their specific capacity, unit area specific capacity and specific capacity index values as suggested by Slichter (1906), Naresimhan (1965), Walton (1962) and Singhal (1973) respectively. The unit area specific capacity is found to be better parameter as compared to others. For the dugwells tapping talus deposits, black cotton soils, alluvium and basalts, it is found to be of the order of 1.2, 0.7 to 1.3, 1.4 and 0.3 to 6.7 (exceptional 25.2) lpm/m/m2 respectively. In the dugwells tapping basalts alone it is found to be of the order of 0.3 to 1.3 lpm/m/m2 for the poorly weathered and poorly jointed basalts, 0.7 to 1.2 lpm/m/m2 for the poorly weathered and highly jointed basalts and 0.9 to 6.7 (exceptional 25.2) lpm/m/m2 for the highly weathered and jointed basalts. On the basis of these parameters, it is concluded that the highly weathered and jointed basalt forms the most potential water-bearing horizon in the area. With a view to classify the water resources of the Koyna sub-basin and assess their suitability both for drinking and irrigation purposes, a total of 147 water samples (76 from dugwells, 24 from borewells, 29 from springs and 18 from the Koyna river) were chemically analysed. Based on modified Hill-Piper diagram (Romani, 1981) the waters from the shallow aquifers are found, in general, to be calcium-bicarbonate type (53%) and calcium-magnesium-bicarbonates type (27%). In case of deeper aquifers they are mostly calcium-magnesium-bicarbonate type (29%), sodium-bicarbonate type (24%), calcium-bicarbonate type (19%), calcium-magnesium-sodium-bicarbonate type (19%) and sodium-calcium-bicarbonate type (9%). The analysis indicate that both the surface and groundwaters are fit for drinking and domestic purposes as per the criteria fixed by Govt, of India (1983). Based on Wilcox diagram (1955), USSL diagram (1954), these waters are also found fit for irrigation purposes. However, in the lower reaches of the Koyna river, the water quality has deteriorated due to use of fertilizers and water-logging conditions. The groundwater in this area shows salinity hazard. With the erection of the Koyna dam, the Koyna sub-basin, stands divided into two parts- Area I (954.20 sq.km) to the north of the Koyna dam which covers the entire catchment area of the dam and Area II (1081.80 sq.km.) downstream of the Koyna dam. These two divisions have defined basin boundaries and have been treated as two separate watersheds. Area I is mostly hilly and the inhabitants mostly depend on spring water while Area II forms relatively plain valley. Groundwater development through dugwells and borewells is mostly confined to Area II only and hence the assessment of the groundwater resources was made for this part of the Koyna sub-basin only. The regional specific yield for Area II has been estimated to be 1.20%. On the basis of water level fluctuation method, the annual groundwater recharge has been estimated to be 60 MCM, out of which 30 MCM is due to monsoon rainfall and the rest 30 MCM is due to induced recharge from the surface water tanks (3MCM) and water applied for irrigation (27 MCM). The safe-yield has been estimated to be 60 MCM, out of which 22 MCM is being used for domestic/stock and irrigational needs, 30 MCM is unutilised baseflow and 3 MCM is spring flow. Thus, there remains a balance of only 5 MCM for further groundwater development. Assuming that at least 25% of the unutilised baseflow (i.e 8 MCM) can be brought to fruitful use, the total amount of groundwater available for further groundwater development amounts to 13 MCM. Taking a unit draft of 0.021 MCM per well per year at the existing hydrogeological set-up, about 620 wells can be constructed in the Command area alone. Similarly, at least 25% of the spring flow (i.e. about 1 MCM) can be tapped through pipe-lines for drinking, irrigation and afforestation. The Koyna sub-basin is characterised by both scarcity and abundance of water-scarcity in the dissected plateau which calls for artificial recharge of the aquifers, and abundance in the command area of the Koyna lift-irrigation schemes which calls for consumptive use of the water resources. For optimal development of the available groundwater resources it is but imperative to practice both these vital aspects. The areas having NE-SW trending lineaments should be looked for greater success of the borewells where more number of dugwells may be constructed mthe deep buried pediments due to their high groundwater poteminl. The command area of the ht,-,rnga„on schemes may be expnnded further to the adjoining scarcity areas for effective development of the ava,lah,e surfuce wa,er potential. The sprmgs form very ,mpor,an, source of wa,er which cou.d be har„essed effecively in ,he hilly ,ra«s without disturbtng ,he ex.sting natural system.